To improve the lift over drag ratio on airfoils, many methods have been used including the attachment of vortex generators and turbulators. These devices delay separation of the boundary layer fluid flow from the surface of an airfoil and thus reduce drag. Reduced drag also provides for an increased usable angle of attack of an airfoil improving lift at slow speeds. There are also many examples of other surface modifications of airfoils including corrugations or other geometrical modifications to modify the boundary layer.
In U.S. Pat. No. 6,431,498, Watts and Fish disclose a scalloped wing leading edge to increase the lift over drag ratio. The wavelike protrusions on the leading edge are aligned such that the channeled air flow on the top side is aligned with the channeled air flow on the bottom side and these flows mix behind the trailing edge. These inventors studied the flippers of the humpback whale as an example of a natural design that provides a hydrodynamic advantage for highly efficient swimming. The whale's flippers are modified by irregular bumps or tubercules across the leading edge that improve lift and reduce drag.
In U.S. Pat. No. 4,813,633 (Werle et. al.) and U.S. Pat. No. 4,813,635 (Paterson et. al.) a wavy trailing edge is used to reduce drag. In these examples, the troughs on the opposite surfaces are aligned. In U.S. Pat. No. 4,830,315 (Presz, Jr. et. al.) a wavy or corrugated trailing edge is used to reduce drag. Likewise in U.S. Pat. No. 5,533,865 Dassen and Hagg and in U.S. Pat. No. 6,830,436, Shibata et. al. describe a serrated saw toothed trailing edge on wind turbine airfoil blades to improve performance as does Gliebe in U.S. Pat. No. 6,733,240 on a turbofan blade. In U.S. Pat. No. 5,114,099, Gao uses a number of surface modifications to reduce turbulent drag of fluids including an airfoil with a partially wavy surface pattern on the top side.
In addition to airfoils for aircraft and wind or water turbines, the technology of boundary layer control is also utilized to improve the performance of heat exchangers. One example of this is U.S. Pat. No. 3,741,285 by Kuethe who teaches the use of surface elements for vortex amplification. Surface elements for wind turbine airfoil blades such as vortex breakers on the trailing edge (US application publication number 2009/0016891) or vortex generators on the top surface (US application publication number 2010/0008787 A1) are just a couple of the many examples of this technology.